Arrangements using multiple sources of power for vehicles, such as electric motor and internal combustion-powered vehicles, often have aspects that fall into one of two general categories, namely those in which the power sources are serially arranged, and those in which they are at least partly parallel. A multiple or hybrid power source, for example, may comprise an internal combustion engine and one or more electric rotating machines operable as motors or as generators in different conditions. These power sources may drive one another serially, or may contribute concurrently (in parallel) to driving the same output shaft.
In a series configuration, a prime mover such as an internal combustion engine powers an electrical generator. The electrical energy thus developed is conditioned by an electrical control unit and subsequently delivered to an electric motor. The electric motor is mechanically connected to apply power to the mechanical driving elements of the vehicle.
A power system as described also may have an associated energy storage means such as a set of storage batteries. The energy storage means can be tapped when needed to provide electric power to the vehicle in addition to the power being produced from the generator. The energy storage means also can be used to store regenerated electrical power collected during braking, e.g., by selectively operating the driving electric motor as a generator, or perhaps using a separate generator. When charged, the energy storage means may provide all the power used to move the vehicle, but only for a limited time and perhaps at reduced performance levels. Similar energy recovery advantages as well as similar alternatives in routing of electrical and mechanical energy can be useful In some instances without relying substantially on storage batteries, for example as in the case of locomotives, where an internal combustion engine may power a generator that is coupled to an electric motor for the purpose of achieving high torque at low speed for starting.
A serial system as described can be effective, particularly for reclaiming vehicle energy by regeneration. One drawback that is inherent in the configuration, however, is that the electric machine(s) that finally couple to the driving elements, determine the limits of performance of the vehicle. For example, the maximum power output developed by the electric motor ultimately determines the maximum acceleration of the vehicle. The electric motor must be specified for sufficient starting torque, steady state loading, with associated heat dissipation capacity and the like, to meet vehicle performance requirements.
In a parallel configuration, mechanical drive is coupled to the driving elements from both the prime mover (e.g., an internal combustion engine) and also from one or more electric motors powered from a generator and/or energy storage means. This combining of available power from two sources is accomplished through a transmission. In such a configuration, both the prime mover and the electric motor can be called upon when needed and their contributions to performance are added. Therefore, a parallel system and a serial system that can achieve a given performance level may differ in that the parallel system uses smaller motors, generators and engines than the serial system to achieve that performance level. This can have a number of positive effects, such as reduced inertial mass, potentially lower energy storage requirements, reduced atmospheric emissions, etc. In a parallel system, the two or more parallel engines, motors and/or generators, can be controlled to contribute differently in different situations, by sequencing, combining and selecting the manner in which the components are coupled to one another and/or to the driving wheels at any given time. Various efficiencies become available.
In such a combined or hybrid unit, a source of mechanical power such as an internal combustion engine, is coupled to a mechanical load that sometimes functions as a regenerative source of power. That mechanical source and load (the load being a source during regeneration) is coupled to at least one electric machine that can likewise can be made to function as a load (during generation of electric power) or as a source when converting electric power to mechanical power. These different functional states of the mechanical and electrical elements provide a number of efficiencies in which mechanical and/or electric power is routed in one direction or another.
Although power may be routed differently, in all the arrangements wherein the internal combustion engine and the rotating electric machine(s) are operating, they conventionally all rotate exclusively in their forward direction. It is possible that a gearing change or vehicle direction can involve a change of direction, such as to move the vehicle in reverse, in which case that direction is the normal operational direction. It is possible to accommodate a specific gearing arrangement through a pair of meshing gears that coupled elements may normally rotate in opposite directions such that the “normal” direction of the engine might correspond to a normal motor/generator direction in on rotational direction or the other. Nevertheless, the “normal” directions of the engine and respective motor/generators are the same in each of the respective operational states of a conventional hybrid apparatus.
For example, the combustion engine could be charging the batteries through an electric generator, or the electric motor/generator could be operating as a motor to add to the mechanical power output by the engine, or the motor/generator might be regeneratively rotated as a generator when recovering vehicle inertia during braking. In each of these situations, the rotating elements conventionally move in their normal rotational direction. This direction is said to be the “engine-wise” direction in this description. The normal or engine-wise direction does not change when power is routed differently. The change in power routing simply concerns which of the devices is producing energy as a source and which is receiving energy as a load.
Parallel-hybrid vehicle transmissions have the capability to couple power from one or more elements functioning as power sources to one or more elements functioning as loads. For example, combustion engine and electric power sharing of this sort can be accomplished by positioning an electrical motor physically between the engine and an existing transmission such that the electric power is additive to the engine power. Assuming that one engine or motor is in line with a driving shaft, the other can be displaced laterally and arranged so that two separate power inputs are supplied. Alternatively, a separate generator can be driven by the engine to power the motor through an electrical circuit, i.e., the arrangement comprising an engine and two electric machines, one normally operating as a motor assisting the engine and the other normally operating as a generator.
Electric machines are operable as motors or as generators depending on whether power is to be collected or expended. Thus power from an engine and a motor can be added together and coupled to the drive wheels. An electric motor can be operated as a generator during braking to recover power. A combustion engine can operate a generator to charge batteries when more engine power is available than is needed for the drive wheels. The battery power can be coupled to one or more electric machines, etc. In the case of two electric machines that may be operated as motors or generators and coupled electrically to a battery, a combustion engine, and the potential to recover inertial energy regeneratively from the drive wheels, including different proportionate contributions of the power sources and the power loads for different conditions, a great deal of flexibility is provided.
Traditional vehicle transmission technology seeks to couple a single power source such as a combustion engine to a single load such as a driveshaft that moves surface-engaging wheels supporting the vehicle. A primary object is to operate the engine over a range of speeds that the transmission converts into different speed/torque combinations at the load. A dual motor/generator parallel-hybrid powertrain system may seek to achieve similar speed/torque combinations at the load, but to do so in a different way, for example wherein the combustion engine is operated at a relatively limited range of speeds where it is most efficient. A conventional transmission (manual or automatic) may be a step-ratio device, having discrete fixed ratios of input speed to output speed, i.e. first gear, second gear, etc.
Another known transmission type is the continuously-variable transmission, or CVT. These transmissions have a minimum low numerical ratio, a maximum high numerical ratio, but continuously variable ratios between these limits. The capability of a continuously variable transmission to assume any gradation of ratios, within limits, can be applied to a dual motor/generator parallel hybrid configuration, where such gradations can be factored into the power sharing arrangements between the components. Hybrid vehicles including the Honda Insight and Toyota Pruis are parallel-hybirds with CVT transmissions.
Small hybrid automobiles may use, for example, a Van Doorne belt CVT configuration, which may be effective for smaller vehicles but is less applicable to large vehicles such as trucks, that need a wide range of ratios for adequate torque multiplication. Providing a ratio adapted for high torque at low speed adversely affects maximum speed characteristics to the point that one “ratio range” may be insufficient. It would be advantageous in those situations to provide successive ratio ranges, i.e., first range, second range, etc.
Multi-range CVT transmissions have been produced on a limited basis, including the Cummins-Sunstrand responder of the early 1970's (a two-range hydromechanical CVT), the HMPT 500 tracked vehicle transmission of the U.S. Army (a three-range hydromechanical example). Four range CVT prototypes have been developed, and hydromechanical CVT agriculture tractors are known. These multi-range hydrostatic powered transmissions set the example for parallel-hybrids.
Recent developments in multi-range CVT parallel-hybrid transmissions have their lineage in hydrostatic drive transmissions as mentioned, but there are differences. One difference is on-board energy storage and switching arrangements whereby two or more electric machines in a hybrid can function as motors or generators, and together with the engine can provide power simultaneously. Since the motor and pump of a hydrostatic device are integrally linked through fluid connections, they cannot provide for separate power inputs or function at different power levels at the same time.
A transmission for automotive applications must be placed in the vehicle without intruding on passenger space or compromising other aspects of design. The mounting position of the major rotating components is a concern, and it may be advantageous to place them in line, on the same axis. A parallel-hybrid transmission based upon a hydromechanical configuration may require a dual axis (at a minimum) with either the hydrostatic input or the mechanical input (or lay shaft) displaced from the center line. Such arrangements are at a disadvantage as to compactness and may require substantial complexity to realize.
Over the variation of possible vehicle size, from subcompact car to large industrial truck, there may be a number of possible mounting and mechanical arrangements possible. However, it would be advantageous if a wide range of sizes could be accommodated using a coaxial or even concentric arrangements where efficiencies can be realized using at least one power source such as a combustion engine and at least one electric machine that can function at least at times in different states as a motor or generator controlled by switching controls to effect different operations.
What is needed is a parallel-hybrid transmission having size/shape and power delivery capabilities comparable to current automatic transmissions, that contributes to the requirements of a hybrid arrangement and enables exploitation of the hybrid's particular advantages.